My first thoughts on seeing this feature in vB5 was - WTF? What sort of idiot would come up with a feature like this?
But, I decided to try & keep an open mind and see if it grew on me, it didn't, it's a mess, it makes it impossible to follow a thread, I can't see how you could make it work in a way that navigation remains easy AND ensures you don't miss both new comments and get to new posts.
The longer a thread is, the worst it becomes, if the 'new post' link takes you to the first 'new comment', you could end-up scrolling dozens of pages before you got to the first 'new post', having to check every post in between to see if further 'new comments' have been added. If the 'new post' link takes you to the first 'new post', you'll miss any 'new comments' made prior to that post, and have to scroll dozens of pages to back try and spot any 'new comments'.
It confuses the hell out of me, complete & absolute nightmare, my original first thoughts on this feature remains.
ETA: If any forum I use ever introduces such a feature, that would be the last day that I would use that forum, no matter how much I had enjoyed the place before.